I decided to build a couple torsion box beams to use as a workbench. I used 3/4 MDF and made the faces of the beams 12” wide. I ran 2 dados on each face, 3/8” thick for the length wise spans to sit in. I glued and clamped the lengthwise spans in place and let dry. I then cut the short spacer peices that fit inbetween the lengthwise spans. I found out though, that my 2 lengthwise spans are not perfectly perpendicular to the face, so none of my small web pieces sit correctly and i now have a beam that’s not flat on the last 2’ of it :(

What did I do wrong? The dados were tight fitting. I used a 3/4” router bit.

Be grateful that MDF is cheap. If you can find either the Norm Abrams video on assembly benches or the David Marks video on assembly benches, watch them. They focus on the need for the platform that you use to make a torsion box to be dead flat and level. If it isn’t neither will your torsion box be flat when it’s finished. If you can’t find either of those two videos, watch every other torsion box video you can find and take the sum of the best practices as you own for the next build. I think in the good ones, you will see the intermediate pieces being checked for square at lots and lots of points before the glue and the top go on. There are other tricks to pay attention to, so take notes.

These torsion boxes remind me more of box beams rather than torsion boxes. Whatever the name is, I would suggest that you use the short pieces during assembly to keep the long pieces perpendicular to the faces. You could also make and use some assembly squares for the same purpose. As indicated in the other comments, a flat and level surface to build upon is important.